This invention relates to measurement of pressures in catheter balloons and is particularly suited for measurement of such pressures in connection with coronary angioplasty catheters.
Coronary angioplasty is a rapidly growing medical procedure. In angioplasty a coronary artery which has become partially blocked by a stenosis (an abnormal narrowing of the artery due to injury or disease) is opened by inflating a balloon carried by a dilatation catheter to the site of the stenosis.
For placement purposes the dilatation catheter carries a movable guidewire which is advanced from the mouth of the respective coronary artery to the area of stenosis in the coronary artery or in one of the branches off the coronary artery. The dilatation catheter is then pushed over the guidewire to the stenosis and is placed so the balloon carried by the dilatation catheter is disposed in the stenotic area.
The balloon, at this time, is collapsed so it fits through the passage through the stenosis. The dilatation or balloon catheter typically has a pair of lumens: one through which contrast media or other suitable inflating fluid flows under pressure to inflate or deflate the balloon, and one through which the guidewire moves. When properly positioned in the stenosis, the cardiologist manually inflates the balloon by forcing contrast media through the inflation lumen into the interior of the balloon. This expansion of the balloon, assuming the size of the balloon has been chosen properly, expands the passage through the blood vessel to something approaching its normal dimensions.
It is often necessary to pressurize the balloon to a pressure in the range of from seven to ten atmospheres and to sustain this pressure for up to thirty seconds or so. During this pressurization, it is desirable to know the pressure to which the balloon is being inflated to ensure that the proper force has been applied to open the passageway. At present this pressure is displayed by an analog pressure gauge disposed in the vicinity of the syringe which is used to inflate the balloon.
Although analog pressure gauges are fairly accurate, they are not particularly easy to read, and are fairly easy to misread. These gauges are also not extremely well suited for integration into a system which can display to the cardiologist other information which would be of interest, such as elapsed time of inflation of the balloon, total desired inflation time, number of times that particular balloon has been inflated, and so on.
During coronary angioplasty the walls of the vessel are stretched and it sometimes occurs that the walls of the vessel fail under the force exerted by the expanded balloon. This incipient failure is reflected in the pressure in the balloon, but analog gauges are not especially well suited to detect this rapid change or to provide some alarm or other indication thereof to the cardiologist so that pressure could be immediately relieved before further damage to the vessel could occur.
In order to determine the actual force exerted by the balloon on the walls of the passageway, prior to the procedure cardiologists sometimes test inflate the balloon in a fixture which measures the applied force. The analog pressure gauge can be observed during this test to correlate the applied force with the pressure reading shown by the analog gauge. Analog gauges, however, are not particularly well suited to automating this test procedure.
Even if the force exerted by the balloon is not tested, the balloon is typically inflated before the procedure to ensure the patency of the balloon and to check whether the balloon can withstand the desired pressure. An analog gauge is typically used during this test to ensure that the desired pressure is applied. Such a gauge, however, is not particularly well suited to automation of this test either.
Although the problems described above arise in the connection with the coronary angioplasty procedure, it should be realized that other medical procedures involving balloon catheters have similar problems. Peripheral angioplasty, for example, is a very similar procedure with similar concerns. Other procedures in which the present invention can be used will no doubt come to mind to those of ordinary skill in the art.
The pressures produced during an angioplasty procedure (the seven to ten atmospheres noted above) are much higher than those (e.g. a person's blood pressure) a hospital is typically equipped to measure. Pressure transducers have been developed for blood pressure measurement which produce an electrical signal which is displayed on electronic digital display equipment or monitors. The American National Standards Institute (ANSI) has issued standards for blood pressure transducers which assure that all manufacturers' equipment will be interchangeable and perform to agreed upon specification. For example, the transducers all have an output of 5 microvolts (uv) per volt excitation per millimeter of mercury pressure (mm Hg), a measurement range of -50 mm Hg to +300 mm Hg, and an overpressure rating of 4,000 mm Hg. Since blood pressure measurement is a standard hospital procedure, personnel are trained in both use of the transducer and the associated electronic monitors.
The generally accepted pressure range of percutaneous coronary angioplasty (PCTA) when measured in psi instead of atmospheres is -15 psi to +300 psi. As can be seen, the numerical range for the blood pressure and PCTA measurements is substantially the same; and, in fact, they differ substantially only in the units of measurement. Consequently, it may be advantageous to have a pressure transducer used for PCTA measurements which meets the ANSI interchangeability standards for blood pressure transducers (except for units of measurement), and thus can be used by hospital personnel, without requiring them to have additional training, with already available electronic digital display equipment. The use of scaling factors such may further be useful with various types of displays, both digital and analog.